A printhead mounted in an inkjet printing apparatus typically includes a nozzle plate which is connected and attached to a printhead substrate (to be referred to as a head substrate hereinafter) at an interval. The nozzle plate includes ink discharge nozzles which are arranged in association with a plurality of printing elements (heaters) attached onto the head substrate. In energizing and driving a specific printing element, ink adjacent to it abruptly expands and bubbles. The bubbling force discharges ink onto a printing medium via the orifice of the ink discharge nozzle.
When a plurality of printing elements (heaters) attached to the printhead are driven, the printhead temperature and ink temperature rise. A change in ink temperature leads to a change in physical characteristic such as ink viscosity or surface tension. The discharge speed of ink droplets discharged onto a printing medium changes along with a change in ink temperature within the printhead. This change influences the printing quality.
Conventionally, in order to maintain ink in the printhead at an almost desired operating temperature, at least one heater (sub-heater) is attached to the head substrate, and the head substrate is heated using this sub-heater or a pulse short enough not to discharge ink is applied to a printing element (heater). This adjusts the printhead temperature, achieving a more uniform, higher printing quality. The sub-heater and printing element (heater) used for temperature adjustment are typically driven in powering on the printhead or while the printhead is idle, so as to maintain ink in the printhead at an almost desired operating temperature.
A conventional printhead using at least one sub-heater typically includes a driver circuit which drives the sub-heater and is separated from a driver circuit for driving a printing element (heater). By using these separated driver circuits, the sub-heater can be selectively driven independently of the printing element (heater), as disclosed in, e.g., U.S. Pat. No. 5,175,565.
However, the arrangement using the sub-heater, the driver dedicated to the sub-heater, and their interconnection circuit, like the above prior art, raises the production cost of the printhead. As a result, the production cost of the printing apparatus which incorporates and controls the printhead becomes high, and the control becomes complicated.
In some cases, printing is also conventionally controlled by using head substrates having no sub-heater as head substrates dedicated to color printing and monochrome printing, and alternately performing color printing and monochrome printing. The temperature is adjusted by natural cooling of a temperature rise caused by driving a printing element.
FIG. 11 is a circuit diagram showing the conventional arrangement of two head substrates dedicated to color printing and monochrome printing.
FIG. 12 is a timing chart showing various signals input to the head substrates shown in FIG. 11.
In the circuit arrangement shown in FIG. 11, printing is exclusively so controlled as to alternately execute color printing and monochrome printing. Thus, a printing signal line (DATA), clock signal line (CLK), and latch signal line (LATCH) are common to a monochrome printing head substrate 100K and a color printing head substrate 100C.
The head substrates 100K and 100C basically have the same arrangement. That is, N printing elements (heaters) 101 are connected to MOS-FET transistors 102 for driving them. The gates of the MOS-FET transistors 102 are connected to the outputs of AND circuits 103. One input of each AND circuit 103 is connected to a heat pulse signal line (ENBK or ENBC), and the other input is connected to the output of a latch circuit 104.
A shift register 106 receives and temporarily stores a printing signal via the printing signal line (DATA) in synchronism with a clock signal supplied by the clock signal line (CLK). When a latch signal is input via the latch signal line (LATCH), printing data is latched by the latch circuit 104 by the next processing.
Another shift register 107 receives a group signal via a group signal line (GRPK or GRPC) in synchronism with a clock signal supplied via the clock signal line (CLK). The group signal is decoded by a decoder 108 into a block selection signal for time-divisionally controlling a plurality of printing elements. The block selection signal is input to one input terminal of each AND circuit, and the other input terminal receives a printing signal from the shift register 106. The latch circuit 104 latches the logical operation result of each AND circuit 105.
As is apparent from FIG. 11, the monochrome printing head substrate 100K and color printing head substrate 100C are connected to dedicated lines as group signal lines and heat pulse signal lines (ENBK and ENBC). This is because the division number and driving order of time division driving are different between color printing and monochrome printing and individual control is necessary.
Each of the head substrates 100K and 100C supports N printing elements. On the color printing head substrate 100C, N/3 printing elements of the N printing elements are used for printing using each of cyan (C) ink, magenta (M) ink, and yellow (Y) ink. In color printing, a color printing signal (CDATA) for a cyan component, a color printing signal (MDATA) for a magenta component, and a color printing signal (YDATA) for a yellow component are sequentially input via the printing signal line (DATA).
In this manner, heat pulse signal lines are separately arranged for the respective head substrates. For example, when monochrome printing is performed using the head substrate 100K, as shown in FIG. 11, a heat pulse supplied via the heat pulse signal line (ENBK) changes to high level, and a heat pulse supplied via the heat pulse signal line (ENBC) connected to the head substrate 100C changes to low level. The printing elements of the head substrate 100C become idle.
For example, when the printhead integrating both the head substrates 100K and 100C is mounted on the carriage of the printing apparatus and the printing apparatus prints while scanning the carriage, color printing and monochrome printing are so controlled as not to overlap each other in the same scanning. In other words, the head substrates 100K and 100C are alternately driven in each scanning to make one of the two head substrates idle. Thus, heat generated by printing operation can be dissipated due to natural cooling.
In FIG. 11, VH represents a driving voltage supply line, and GND represents a ground line.
In this arrangement, the use of common signal lines can simplify the circuit arrangement, but the temperature cannot be intentionally adjusted. The problem of temperature control cannot be fully solved.
In the arrangement in which printing is exclusively performed for each substrate using common signal lines, a heater which is controlled independently of an arrangement used for printing must be arranged on the head substrate in order to adjust the temperature by heating on a head substrate which does not print. This increases the head substrate area, and the cost rises due to a large area.